Many video coders support an inter-frame coding mode in which movement between successive images of a video sequence is estimated in order for the most recent image to be coded relative to one or more preceding images. Movement in the sequence is estimated, the estimation parameters being sent to the decoder, and the estimation error is converted, quantized, and sent to the decoder.
Each image of the sequence can also be coded without reference to the others. This is known as intra-frame coding. This coding mode exploits spatial correlation within an image. For a given bit rate for transmission from the coder to the decoder, it achieves lower video quality than inter-frame coding because it does not make use of temporal correlation between the successive images of the video sequence.
A video sequence portion routinely has its first image coded in intra-frame mode and subsequent images coded in intra-frame mode or inter-frame mode. Information included in the output stream from the coder indicates the macroblocks coded in intra-frame mode and in inter-frame mode and, for inter-frame mode, the reference image(s) to use.
A problem with inter-frame coding is its behavior in the presence of transmission errors or loss of packets on the communication channel between the coder and the decoder. Deterioration or loss of an image propagates to subsequent images until a new intra-frame coded image arrives.
It is routine for the mode of transmission of the coded signal between the coder and the decoder to generate total or partial loss of certain images. For example, with transmission over a packet network having no guaranteed delivery, such as an IP (Internet Protocol) network, such losses result from the loss or the delayed arrival of certain data packets. Losses can also result from errors introduced by the transmission channel that exceed the correction capabilities of the error corrector codes employed.
In an environment subject to various signal losses, it is necessary to provide mechanisms for improving image quality in the decoder. One of these mechanisms uses a back channel from the decoder to the coder on which the decoder informs the coder that it has lost some or all of certain images. The drawback of this is that:                the information is not specified temporally, i.e. the number of the image is not known;        the information is not specified spatially, i.e. the decoder makes no distinction between image portions that have been received and image portions that have been lost.        
Following reception of this information, the coder makes coding choices to correct or at least reduce the effects of the transmission errors. Current coders simply send an intra-frame coded image, i.e. one with no reference to the images previously coded in the stream that may contain errors.
These intra-frame coded images are used to refresh the display and to correct errors caused by transmission losses. However, they are not of such good quality as inter-frame coded images. Thus the usual mechanism for compensating image losses leads in any event to deterioration of the quality of the reconstructed signal for a certain time after the loss.
There are also known mechanisms in which the decoder is capable of signaling lost image portions to the coder in more detail (better spatial and temporal location). For example, if during processing of an image N by the decoder the decoder determines that the macroblocks i, j, and k of the image N have been lost, the decoder then informs the coder of the loss of those macroblocks. Such mechanisms are described in the following documents in particular:                IETF/AVT Draft, “Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)”, Ott, Wenger, Sato, Burmeister, Rey, August 2004;        ITU-T Video Coding Experts Group (ITU-T SG16 Q.6);        VCEG-X09, “Introduction of a back channel for H.264”, Loras, October 2004;        ITU-T Video Coding Experts Group (ITU-T SG16 Q.6)        
VCEG-Y15, “Definition of a back channel for H.264: some results”, Baillavoine, Jung, January 2004;                ITU-T, “Control protocol for multimedia communication, Recommendation H.245”, July 2003.        
The drawback of this type of mechanism is the absence of reaction and therefore of processing by the coder following reception by the coder of information to the effect that image portions have been lost.